Liposomes, which are composed of naturally occurring lipids have been used in the delivery of various agents within an organism. Recent research has been directed to synthetic compositions, such as polymersomes, that function similarly in some respects to liposomes. Certain polymer systems, like lipids, spontaneously form closed structures when their building blocks are placed in aqueous medial. Polymersomes, however, are vesicles made using amphiphilic diblock and multiblock copolymers where at least one block is hydrophobic and at least one block is hydrophilic. In particular, these diblock and multiblcok copolymers can form thick-walled vesicles when placed in an aqueous media. Polymersomes can be stably prepared by a number of techniques which are common to liposomes (Lee et al., Biotechnology and Bioengineering, vol. 73, no. 2, Apr. 20, 2001). Processes such as film rehydration, sonication, and extrusion can generate many-micron giant vesicles as well as monodisperse vesicles with diameters as small as 100 nanometers.
Certain vesicles of PEO-PEE (polyethyleneoxide-polyethylethylene) or PEO-PBD (polyethyleneoxide-polybutadiene) are known to form thick-walled (e.g., about 100 nm thick) vesicles that exhibit improved stability compared to liposomes. For example, a PEO-PEE diblock introduced by Hillmeyer and Bates (Macromolecules 1996; 29:6994-7002), specifically EO40-EE37 (designated OE7, where EO is ethylene oxide monomer and EE is ethylethylene monomer), has been shown to self-assemble into membranes that are hyper-thick compared to any natural lipid membrane and are also an order of magnitude or more tougher, and thus, show greater mechanical stability (Discher B M, Won Y-Y, Ege D S, Lee JC-M, Bates F S, Discher D E, Hammer D A., Science 1999; 284:1143-1146). Augmented chemical stability due to their polyethylene-oxide (PEO) head groups confers biocompatibility, structural integrity in plasma, and “stealth”-like character resulting in long in vivo circulation times (P. J. Photos, L. Bacakova, B. Discher, F. S. Bates, D. E. Discher, Journal of Controlled Release Jul. 31, 2003; 90, 323-334;). A novel PEO-PBD diblock, EO26-BD46 (designated OB2, where EO is ethylene oxide monomer and DB is butadiene monomer)), has also been shown to be capable of making vesicles. Both OE7 and OB2 have mean molecular weights in excess of several kda—much larger than any natural membrane-forming amphiphile.
Optical-based methods constitute new and attractive in vivo imaging modalities due to their impressive potential spatial resolutions, the inherent biological safety of low-energy fluorescent light, and the continuing development of cheap and mobile excitation and detection sources (R. Weissleder, Nature Biotechnology. 19, 316-7 (2001)). Although visible probes enable imaging of live animals by intravital microscopy (R. K. Jain, L. L. Munn, D. Fukumura, Nature Reviews. Cancer. 2, 266-76 (2002)), their utility is significantly limited at greater than sub-millimeter tissue depths due to excessive light scattering and optical absorption. Because light scattering in the visible spectrum diminishes with the reciprocal of the fourth power of wavelength (λ−4), and hemoglobin electronic and water vibrational overtone absorptions approach their nadir over the near infrared (NIR) spectral domain (700-950 nm) (B. Chance, in Advances in Optical Biopsy and Optical Mammography. (1998), vol. 838, pp. 2945), considerable effort has been spent developing systems that utilize NIR light for in vivo imaging applications (V. Ntziachristos, A. G. Yodh, M. Schnall, B. Chance, Proceedings of the National Academy of Sciences of the United States of America. 97, 2767-72 (2000) and R. Weissleder, V. Ntziachristos, Nature Medicine. 9, 123-8 (2003)).
Molecules such as porphyrins, chlorophylls, purpurins, tetrapyrroles, macrocycles based on polypyrrole structures and their metalated derivatives, as well as fullerenes can act as imaging agents for target tissues such as tumors. Administration of these agents to an organism results in preferential localization of the agent in any of a variety of pathologies with respect to surrounding tissue. Irradiation of the organism with light of a particular wavelength(s) can lead to absorption of light by the agent, and in some cases, fluorescence or phosphorescence by the agent. This absorption/emission process causes contrast between the target tissue and the surrounding tissue. Detection of this contrast allows imaging of the targeted tissue.
While significant progress has been made in constructing target-specific and locally-active NIR-emissive probes (R. Weissleder, C. H. Tung, U. Mahmood, A. Bogdanov, Jr., Nature Biotechnology. 17, 375-8 (1999); S. Achilefu, R. B. Dorshow, J. E. Bugaj, R. Rajagopalan, Investigative Radiology 35, 479-485 (August, 2000); A. Becker et al., Nature Biotechnology. 19, 327-31 (2001); and A. Zaheer et al., Nature Biotechnology. 19, 1148-54 (2001)), the development and delivery of contrast agents of appropriate sensitivity remains a major technological hurdle for the realization of fluorescence-based molecular imaging in deep tissues.